1. Field of the Invention
The present invention relates to a radio communication apparatus, and a method and program for error correction. More particularly, the present invention relates to a radio communication apparatus having an error correcting function, and a method and program for error correction.
2. Description of the Background Art
With advance in communication systems, error correction has been increasingly adapted for the purpose of improving communication qualities. A method for error correction is described, e.g., in Hideki Imai, “Code Theory”, The Institute of Electronics, Information and Communication Engineers, pp. 24-25, Mar. 15, 1990.
Using the error correction enables accurate recovery of a transmitted signal from a received signal under a certain condition in such a case that noise occurs in the communication path, causing a difference between the bits of the received signal and the transmitted signal.
Error correction, however, not only provides the above-described benefits, but may also cause detriments as follows.
In the error correction, a transmitted signal is estimated from a received signal. From the standpoint of a space of the received signal, the space of the received signal is divided in advance into regions corresponding to the respective transmitted signals, as shown in FIG. 13, and the transmitted signal is estimated by checking in which region the received signal has entered.
Referring to FIG. 13, the region denoted by (1) is a region (correction region for signal point A) corresponding to the transmitted signal of signal point A. The received signal having entered this region is estimated to be the signal of signal point A by conducting error correction.
The region denoted by (2) is a region (correction region for signal point E) corresponding to the transmitted signal of signal point E. The received signal having entered this region is estimated to be the signal of signal point E as a result of error correction.
The shaded region in FIG. 13 is a region (error correction failure region) corresponding to none of the transmitted signals. It is impossible to perform error correction on the received signal having entered this region.
FIG. 14 shows presence/absence of an error in a received signal and success/failure of recognition of the error, in the case where a signal of signal point A transmitted is received at respective signal points A-E of FIG. 13, for two separate cases of performing and not performing error correction.
In FIG. 14, the symbol ∘ indicates that a correct signal is obtained, x indicates that an incorrect signal is obtained and the error is recognized, and Δ indicates that an incorrect signal is obtained and it is falsely recognized that there is no error.
When the signal point of the received signal corresponds to signal point A, a correct signal (signal of signal point A) is obtained whether or not error correction is conducted.
When the signal point of the received signal corresponds to signal point B, it is recognized that there is an error, and an incorrect signal is obtained unless error correction is conducted. Since signal point B is included in the correction region for signal point A, a correct signal (signal of signal point A) can be obtained by conducting error correction.
When the signal point of the received signal corresponds to signal point C, it is recognized that there is an error. A correct signal cannot be obtained, since signal point C is included in the error correction failure region where error correction cannot be performed.
When the signal point of the received signal corresponds to signal point D, it is recognized that there is an error and an incorrect signal is obtained when error correction is not conducted. Since signal point D is included in the correction region for signal point E, when error correction is conducted, an incorrect signal (signal of signal point E) is obtained and it is falsely recognized that there is no error.
When the signal point of the received signal corresponds to signal point E, if error correction is not conducted, an incorrect signal (signal of signal point E) is obtained and it is falsely recognized that there is no error. Even if error correction is conducted, since signal point E is included in the correction region for signal point E, an incorrect signal (signal of signal point E) is obtained and it is falsely recognized that there is no error.
In the above description, the state indicated by o (where a correct signal is obtained) is called “correction success”, and there is no problem.
The state indicated by x (where an incorrect signal is obtained and it is recognized that there is an error) is called “correction failure”. In this case, the presence of an error is recognized, and thus, the reception device can refrain from conducting processing based on the erroneous signal, so that there is no adverse effect on the processing of the reception device. The reception device may instruct the transmission device to retransmit the signal, and obtain a correct signal from the retransmitted signal.
The state indicated by Δ (where an incorrect signal is obtained and it is falsely recognized that there is no error) is called “miscorrection”. The reception device performs processing based on the falsely corrected signal, which may adversely affect the processing of the reception device.